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Prevention Dr Raya Al-Naimi
Lect. 12
Mode of action of fluoride or mechanism of caries control of
fluoride
Fluoride act in several different ways to reduce dental caries
action and this multiplicity of effect is the key to this caries
prevention action.
1- Enamel crystal structer or change of the tooth structer
during the tooth formation.
The principle mineral substance in enamel(also dentin and bone) is the
hydroxy apatite HA, however the fluoride ions has a strong affinity for
mineralized tissues and hydroxy apatite can be readily converted in to
flouro apatite when fluoride has replaced hydroxyl group and this takes
place during period of formation and calcification of tooth. The relative
proportion of FA to HA in enamel varies , with a higher proportion of FA
in outer enamel in enamel exposed to F sol or covered with plaque for a
long period.
A- The HA crystals tended to posses voids and such that voids were
likely to increase crystal reactivity, so they dissolve more readily and
easily , the F ion eliminates the voids and stabilizes the crystal structer ,
so enamel with FA resist to demineralization and dissolution of enamel.
B- Apitite crystals tend to be surrounded by a hydration layer which may
include other ions such as magnesium carbonate and fluoride. But
magnesium and carbonate , unlike F are associated with poor
crystallinity and a further favorable reaction of F would appear to be the
ability to replace magnesium and carbonatein the apitite crystals so
making the crystal more stable.
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C- Incorporation of F also tend to increase the size of the crystal, this
would result in a decrease in crystal surface area per unit volume and
therefore reduce the rate of enamel dissolution.
So the increase in F concentration in enamel is either while the apitite
crystals of the tooth are forming (systemic F administration ) or after the
enamel is formed and the tooth has erupted from the mouth ( topical
administration ), it has been found that maximum cariostatic effect will be
reached when about 50 % of OH positions are reoccupied by F.
2- Bacteriostatic Effect of Fluoride
The F has inhibition effect on enzyme especially glycolytic enzyme like
(enolase) inhibit glycolysis , the process by which sugar is metabolized
by bacteria to produce acid .
There is also some evidence that F inhibits the production of extra
polysaccharide (dextran) by cariogenic bacteria , necessary process for
plaque adherence to smooth surfaces.
A further possible mechanism , there is evidence that F may have specific
bactericidal action on cariogenic bacteria in the plaque especially in
higher concentaration and at a low PH.
The F concentration necessary for reduction of acid production it show
that even a low conc. Of fluoride 1 – 2 ppm of F are able to produce
detectable reduction in acid production , 10 ppm produce a moderately
large reduction in acid production and 100 ppm completely inhibits
bacterial growth.
The amount of available F is several times greater in F rich then in low F
area in dental plaque, so the bacteria of dental plaque formed in
fluoridated area when exposed to levels of F that can
inhibit further
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acid formation , were as those of plaque formed in non fluoridated area
are not , thus the PH values of plaque formed in low fluoride plaque is
much less , so loss of enamel mineral occurs when the plaque is low in F
and this loss is reduced when plaque F level is high.
3- Remineralization
It may be defined as a deposite of mineral or inorganic substances in
an area from which such substances were previously removed.
In early stages of carious lesion (chalky) the lesion is not simply a
process of demineralization, but is characterized by alternating period of
destruction and repair.
During the carious process of demineralization of enamel , apitite is
reduced to simpler compounds or ions , but during a subsequent
remineralization phace the apitite will again be formed .
Apitite is the most stable and least soluble of biological calcium
phosphate compounds and its formation during mineralization phase is
therefore desirable, one of the most important actions of fluoride is its
ability to increase the formation of apitite during remineralization .
A researcher suggested that during remineralization the outer layer of
the apitite crystals may take up fluoride so that it acquires the property of
fluoroapitite .
Possibly due to fluoride ability to inhanse remineralization of carious
enamel , fluoride becomes incorporated in to the new crystal structer
increased amounts over many years .
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Epidemiological evidence of remineralization invivo
Many studies noted that white spot lesion of caries may become
arrested or retarded under favorable oral conditions.
In a study , the examination of 184 buccal surfaces of maxillary first
molars in the same children at age 8 and again at age of 15 years , of 72
surfaces with white spots at 8 years , 37 appeared sound at the age of 15 .
Evidence of remineralization from experimental caries in
man
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When blocks of enamel , covered with Teflon gauze to facilitate
bacterial colonization were mounted in removable partial denture in the
human mouth , the enamel blocks showed surface softening as detected
by microhardness test ,when the gauze was removed to expose the enamel
to the saliva invivo an increase in hardness occurred in the block, this
increase in hardness was thought due to remineralization.
Recently an experiment using the same method of producing
experimental caries, a fluoride solution was used to encourage
remineralization , subsequently the healed lesion were again covered with
gauze to create a cariogenic environment, the enamel which had
remineralized was now found to be more resistant to dental caries then
adjacent area of sound enamel it thus seems possible that a remineralized
white spot lesion may be more resistant to carious attack than sound
enamel.This cycle could occure many times , each time resulting in
increased concentration of fluoride in the new enamel and a reduced
susceptibility to future caries attack.
Other experiment, the subject used sucrose mouth rinse for 23 days to
induce caries after this time oral hygiene procedures were recommended
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and subjects used daily mouth rinses with 0.2 % NaF solution for 1
month, caries scores showed regression or healing of the experimental
lesion during the F mouth rinsing regieme.
Remineralization evidence invitro
In this technique , the researcher used microhardness testing to measure
the quantitive change in enamel surface. In these experiments , first the
sound human teeth were tested for hardness and then tested after
softening with an acetate buffer solution and retested again after exposure
to remineralizing solution and saliva which contained calcium , phosphate
and fluoride ions , in this experiment it was assumed that the rehardening
of the enamel surface was indicative for remineralization .
4- Alteration in morphology of teeth
Teeth in fluoride areas had shallow fissure and well developed well
rounded cusps and some times where 2 % smaller.
5- Delayed eruption of teeth in fluoridated areas
Little support for this hypothesis.
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How do Topical Fluoride work
Several mechanisms have been postulated and it is entirely
probable that a combination of these works together to arrest a
preventive effect.
1- Formation of fluorohydroxy apitite which cause the enamel to become
more resistant to dissolution by bacterial acids ( most common
hypothesis)
2- Fluoride enriched enamel which begins to dissolve in bacterial acids
liberates a quantity of fluoride sufficient to retard the demineralization
process and to promot remineralization.
3- The frequent application of topical fluoride can reduce the relative
quantity of cariogenic microorganism in plaque especially Strepto coccus
mutans ,this is due to direct action of topical fluoride agent on bacteria or
due to indirect rout involving fluoride released from enamel or plaque as
a result of bacterial acid accumulating.
4- The less important suggestion is that fluoride treatment of the teeth
will lower the enamel free surface energy to make plaque accumulation
more difficult or could even desorb bacteria from hydroxyapitite.